THE EXCRETION OF URINE 



543 



of the capsule varies with the activity of the kidney, as is shown in 

 the following figures given by Brodie and Mackenzie. 1 



The urinary tubule has jLj-emarl^able blood supply. The renal arteries 

 arise directly from the abdominal aorta and are very short. They run 

 through the medulla to the cortex, and join with neighboring arteries to 



B. 



Fig. 171. Cross sections of convoluted tubules from kidney of rat. A, during slight secretion; B, 

 during maximal secretion. (From Sauer.) 



form arches from which proceed branches, that radiate into the cortex 

 and give off smaller branches each of which very shortly breaks up into a 

 small capillary tuft, the glomerulus, which lies in the invaginated sphere 

 of Bowman's capsule. By concentration of a beam of light on the kidney 

 of the living frog Richards and his co-workers have succeeded in rendering 

 the capillary blood flow in the glomeruli readily visible under the 

 microscope. The capillaries collect into an efferent vessel, which ap- 

 pears to be smaller than the efferent artery, and this vessel in emerging 

 from the capsule again breaks up to form a capillary network about the con- 

 voluted tubules, forming their sole blood supply. These capillaries 



